This invention relates to master compositions for refining the primary silicon grains of hyper-eutectic silicon-aluminum casting alloys.
Hyper-eutectic silicon-aluminum alloys (i.e., alloys containing more than 12.5 percent silicon), to be suitable for casting, require refinement of the primary silicon grains. Heretofore, phosphorus, in the form of 7 to 15 percent phos-copper has been the conventional agent for achieving this purpose. However, phos-copper has the disadvantage of high cost, particularly when added to hyper-eutectic silicon-aluminum which has been prealloyed with the commonly required percentage of copper so that no credit can be taken for the copper added with the phos-copper grain refiner. Another disadvantage inherent in the use of phos-copper is the fact that the copper specification for the hyper-eutectic silicon-aluminum alloy might be exceeded upon one or more remeltings which require further grain refining by the addition of phos-copper after each remelting.
Phosphorus-silicon master compositions containing 1 to 10 percent phosphorus, formed by adding ferrophosphorus to molten silicon, have also been used as grain-refiners for hyper-eutectic silicon-aluminum casting alloys. Master compositions of this type have good "holding power", i.e., continued refinement of the primary silicon grains of the casting alloy even after remelting. However, phosphorus-silicon master compositions have not been found to be commercially useful, mainly because they give off highly toxic phosphine when exposed to moisture in the ambient atmosphere.
A need has therefore existed in the hyper-eutectic silicon-aluminum casting alloy field for a grain-refiner which is free of the foregoing disadvantages.
Accordingly, it is an object of the present invention to provide an improved phosphorus-containing master composition for refining the primary silicon grain structure of hyper-eutectic silicon-aluminum casting alloys which does not require the use of copper as a constitutent of said master composition.
Another object is to provide an improved phosphorus-containing master composition for refining the grain structure of hyper-eutectic silicon-aluminum casting alloys which is not subject to the formulation of phosphine when the master composition is exposed to moisture in the ambient atmosphere.
Yet another object of the invention is to provide a process for refining the primary silicon grain structure of hyper-eutectic silicon-aluminum casting alloys by the addition to said alloys of a phosphorus-bearing master composition without the co-addition of copper and which is not attended by the evolution of phosphine.
These and other objects of the invention as well as a fuller understanding of the advantages thereof can be had by reference to the following description and claims.